1. Field of the Invention
This invention relates generally to a control system for a hybrid vehicle provided with a plurality of prime movers for running a vehicle, and more particularly, to a control system for a hybrid vehicle which has an electric oil pump for generating an oil pressure to set a torque capacity of a power transmission line.
The disclosure of Japanese Patent Application No. 2004-14320 filed on Jan. 22, 2004 including specification, drawings and claims is incorporated herein by reference in its entirety.
2. Discussion of the Related Art
One example of the hybrid vehicle equipped with a “mechanical distribution type drive unit” is disclosed in Japanese Patent Laid-Open No. 2002-225578, and a construction thereof will be described briefly hereafter. In the disclosed hybrid vehicle, an engine torque is inputted to a carrier of a single pinion type planetary gear mechanism which constitute a distributing mechanism, a first motor generator is connected to a sun gear, and an output member such as a counter drive gear etc. is connected to a ring gear. A second motor generator is connected to the output member or the ring gear through a transmission. The transmission is capable of switching a gear stage between a direct gear stage in which the entire transmission rotates integrally, and a low gear stage in which an output speed is lower than an input speed. Those gear stages are set by properly operating an engagement mechanism operated by the oil pressure.
The hybrid vehicle of this kind can be run not only by a motive power of an engine and the first motor generator, but also by using a torque outputted from the second motor generator as an assist torque. Otherwise, it is also possible to be run only by an output torque of the second motor generator.
Also, in Japanese Patent Laid-Open No. 2000-230442, there is disclosed one example of a hybrid vehicle, which is constructed to maintain driving of an engine-driven oil pump by detecting a trouble of an electric pump from a cumulative load of the electric oil pump and a racing condition of the engine, and by inhibiting a halt of the engine when the electric oil pump is under abnormal condition.
In order to secure the oil pressure when the engine is halted, in the hybrid vehicle disclosed in Japanese Patent Laid-Open No. 2002-225578, it is conceivable to further provide an electric hydraulic pump which is capable of generating the oil pressure even when the engine is halted, in addition to a hydraulic pump which is driven by the engine.
In case of running this hybrid vehicle by an electric motor, output torque of the electric motor is transmitted to an output shaft through a transmission. However, since an engine is halted, it is necessary to set the transmission to a predetermined torque capacity by feeding an oil pressure from the electric oil pump. Therefore, in case of running the vehicle as an electric vehicle (i.e., EV running) by an electric motor, if the electric oil pump has some sort of trouble so that an output pressure or a discharge amount of the oil pressure is insufficient, the torque capacity of the transmission becomes zero or insufficient. As a result of this, the output of the electric motor cannot be transmitted to the output member, and this may prevent the running of the vehicle. In addition, the system suggested in Japanese Patent Laid-Open No. 2000-230442 is a system to secure the torque capacity of a transmission for transmitting an engine torque to an output shaft, and this system cannot be applied to hybrid vehicles in which a prime mover other than an engine is connected to the output member through the transmission.